Hydrocarbon conversion processes, for example steam cracking, visbreaking, catalytic cracking and coking, are carried out at high temperatures to produce a wide variety of olefinic compounds such as ethylene, propylene, n-butene-1, n-butene-2 compounds, isobutene or pentenes, and diolefinic compounds such as 1,2-propadiene, 1,3-butadiene and other compounds with boiling points in the "gasoline" cut range and which can be olefinic or diolefinic. Such processes inevitably lead, however, to the formation of highly unsaturated compounds such as diolefins (for example 1,2-propadiene), also alkynes (for example acetylene, propyne, 1-butyne, etc.). Such compounds have to be eliminated to allow the different cuts from those processes to be used in the chemical industry or for polymerisation processes. As an example, the C.sub.4 cut from steam cracking contains a high proportion of 1,3-butadiene, butene-1, butene-2 compounds and isobutene.
Conventionally, butadiene is separated from the olefinic cut, for example by extractive distillation in the presence of dimethylformamide or N-methylpyrrolidone. The olefinic cut thus obtained contains isobutane, isobutene, butene-1, butene-2 compounds, n-butane and 1,3-butadiene, the latter being in an amount which can be between 0.1% to 2% by weight.
If butadiene is not an upgraded product, the cut can be directly treated using catalyst in the presence of hydrogen to transform the butadiene into n-butenes.
If the butene-1 and isobutane are desired products, processes must be used which produce a large quantity of butene-1 and separate different compounds, such as selective hydrogenation of butadiene to butenes with a small amount of isomerisation of butene-1 to butene-2, or separation of the isobutene by etherification with methanol to produce methyl-tertiobutyl ether.
There is currently a large demand for butene-1. This compound is used as a monomer in the polymer industry. Such use necessitates almost complete hydrogenation of butadiene, the presence of which is only tolerated in amounts of less than 10 ppm by weight.
Attaining these low butadiene contents with conventional catalysts based on nickel or palladium means a reduction in the butene-1 content due to butane formation and isomerisation of butene-1 to butene-2. In order to inhibit isomerisation of butene-1 to butene-2 compounds, some bimetallic formulae comprising palladium and a different metal have been proposed. In particular, palladium-silver systems can be cited, such as those described in U.S. Pat. No. 4,409,410, or palladium-gold, palladium-zinc, palladium-copper, palladium-cadmium, or palladium-tin systems, such as those described in Japanese patent application JP-A-87/05 4540. Proposed solutions for limiting consecutive hydrogenation, and thus butane formation, are more limited. As described in the literature (see, for example, "Selective Hydrogenation Catalysts and Processes: Bench to Industrial Scale", J. P. Boitiaux et al., in "Proceedings of the DGMK Conference". Nov. 11-13, 1993, Kassel, Germany), the hydrogenation selectivity for converting highly unsaturated compounds (diolefins and acetylenic compounds) to olefins originates from considerable complexation of the unsaturated compound on the palladium, preventing the olefins from accessing the catalyst and thus preventing their transformation to paraffins. This is clearly illustrated in the publication cited above were 1-butyne is selectively transformed to butene-1 on a palladium based catalyst. However, it should be noted that the rate of hydrogenation is relatively low. When all of the acetylenic compound has been converted, butene-1 hydrogenation is carried out at a much higher rate than the hydrogenation of the acetylenic compound. This phenomenon is also encountered with selective hydrogenation of butadiene.
This phenomenon poses several problems in industrial units. Firstly, in order to meet specifications regarding butadiene in the olefinic cut, a large quantity of butene-1 is transformed to butane since when the residual concentration of butadiene is low, the hydrogenation rates of butadiene and butene-1 are close to each other. Developing a catalyst which can allow butadiene hydrogenation at a rate which is much higher than the rate of butene-1 hydrogenation, whether these compounds are alone or mixed, is thus very important. This corresponds to catalyst properties which allow hydrogenation with high rate constant ratios for the hydrogenation of butadiene over that of butenes.
The importance of such a catalyst is not limited to an increase in butene-1 selectively but it can also allow better control of the hydrogenation process. In the event of minor local hydrogen distribution problems, using such a catalyst would not lead to high conversion of butenes to butane and would thus minimise the problems of high exothermicity linked to these poorly controlled hydrogenation reactions which would aggravate distribution problems.
To solve this problem, it was important to develop a hydrogenation catalyst which could hydrogenate 1,3-butadiene to butenes while inhibiting the isomerisation of butene-1 to butene-2 and which was less active for consecutive hydrogenation of butene-1 to butane.